An engineering study of altitude determination deficiencies of the Service Aircraft Instrumentation Package (SAIP)

Altitude determination errors of the U. S. Navy's Service Aircraft Instrumentation Package (SAIl'), an airborne positioning pod, were examined in a multifaceted study involving in-flight evaluations, wind tunnel testing and pressure sensitivity experiments. The original objectives of the research' related to identifying aerodynamic sources of pod static pressure inaccuracies and recommending specific remedies to alleviate these errors. After an extensive evaluation, results revealed that.the problem exists not in the aerodynamic measurement performance of the probe, but in the electronic circuitry residing within the Air Data Unit (ADU). The ADU houses mUltiple pressure transducers, each associated with different static and dynamic pressure ports, in a single module. This circuit configuration leads to electrical interference and an attendant degradation of the static pressure output voltage. Accurate static pressure voltages, which can be subsequently converted into appropriate SAIl' barometric altitudes, are obtained by electrically isolating the three ADU dynamic pressure transducers from the single operative static pressure transducer and remaining ADU circuitry.